Dave is ever alert. Even so, sometimes he is surprised by things. No matter how wary Dave is of his surroundings, he isn't a Supercat. He has his limitations. One of those limitations is his peripheral vision. As good as it is, he can't see the whole room at the same time. He is not a fish with eyes on either side of his head. That would be ridiculous. This means that sometimes Dave will be perusing one side of the room, watching out for good napping locations or any potential threats, and another threat could confront Dave from the other side of the room.
In the photograph seen below, Dave thought he had been sneaked up on by some ne'er-do-well. In fact, it was merely a person with a camera, posing no threat at all to Dave. Moments after the photo was taken, Dave would appeared more relaxed, although somewhat annoyed.